1. Field of the Invention
This invention relates to radio frequency (RF) transponder systems, and more particularly to transponders with antenna coils on opposites sides of a card-like substrate.
Description of the Related Art
RF transponder systems are used to monitor for the presence of an object associated with the transponder, to identify the object, and/or to communicate various types of information about the object back to a receiving station. Such systems typically employ an exciter/reader that transmits an RF excitation signal, and a transponder that is energized by the excitation signal to transmit a signal indicating its presence, an identification code and/or other information back to the exciter/reader.
One type of transponder uses an antenna formed on a thin dielectric card such as plastic or paper, mounted with an integrated circuit (IC) transponder chip. Typical applications include transponder tags on clothing and other merchandise that must pass through an exciter reader station to exit the store, thus providing an anti-theft function, airline baggage identification, and security access in general. Flat transponder cards can be used in many applications where bar codes and magnetic strip tags are presently used.
U.S. Pat. No. 4,583,099 to Reilly et al. discloses a transponder card with antenna coils on its opposite sides that are aligned with each other so as to produce a capacitance between the two windings that eliminates the need for a discrete capacitor in the resonant circuit. However, the circuit requires a relatively large winding for a given amount of inductance. It produces a "here-I-am" indication when excited by a signal from an exciter/reader, but beyond that does not provide any identification of the particular article to which it is attached.
U.S. Pat. No. 4,598,276 to Tait described another "here-I-am" transponder card in which a pair of aligned coils are provided on opposite sides of the card and connected through the card at their inner end. The opposed coils are divided into sections, the ends of which are shorted to each other through the card, thus producing discrete inductance and capacitance sections. The patent asserts that, by connecting predetermined portions of the opposing spirals together, the effective capacitance is increased over that resulting if the respective inductors are only capacitively coupled. The circuit's resonant frequency is correspondingly decreased, while the inductance of each loop is not appreciably changed. Similarly current inductively coupled into the connected spirals iS altered, depending on which portions of the spirals are connected. The result is said to be distributed capacitances that reduce eddy currents, thus obtaining a high Q factor and desirable signal emitting characteristics. However, like U.S. Pat. No. 4,583,099, a relatively large winding area is required for a given inductance, and no unique identification or similar information is provided.
The described patents operate by pertubating a transmitted RF signal, rather than generating unique signals for transmission back to the exciter/reader to provide an identification or other information. For transponders having identification or other information transmitting capability, it is particularly important to have an efficient mechanism for both exciting the transponder, and for generating and transmitting back its return information signal. Inefficiencies in transponder operation in either signal direction can limit the system's range, introduce errors and restrict the range of information that can be transmitted.